According to this article in the New York Times home builders are adjusting to new family dynamics by offering more options:
Lennar started marketing its new designs last fall with particular gusto: “Next Gen — The Home Within a Home” is a title and tag line intended to wrap the notion of multigenerational living in a futuristic gloss. But it is more than just marketing; the blueprints themselves are changing.
In fact, architectural historians, statisticians and builders themselves are pointing out that the new household — and the house that can hold it — is much like the old household, the one that was cast aside after World War II by the building boom that focused on small, tidy dwellings for mom, dad and their two children.
Population statistics help tell the tale. A Pew study reports that 41 percent of adults between 25 and 29 are now living, or have lived recently, with their parents. Over all, more than 50 million Americans are in multigenerational households, a 10 percent increase from 2007. It is a back-to-the-future moment.
Of course zoning laws that were put into place in the era of uniform single family home development can make building this type of housing difficult:
Mr. Litchfield warns of the “dead hand of single-family zoning” that inhibits the formation of households like those headed by a single mother, who needs to rent out space to make ends meet, or by boomers who want to build an accessory dwelling unit for an aging parent. Only California, he added, has a state law that allows homeowners to build such a unit “by right,” though some cities have altered their zoning codes to encourage their creation.
To circumvent zoning that is leery of duplexes, Lennar’s Next Gen houses run on a single electric meter, have only microwave convection ovens in the apartment, and from the outside look like other houses.